The present invention relates to metallurgy of refractory rare-earth metals, and more particularly, to niobium metallurgy, and is useful in the production of high-purity niobium and articles made thereof for microwave technology and microelectronics.
Very stringent requirements are imposed on the purity of materials used in the aforementioned fields (total amount of impurities shall not exceed 0.01 wt % or 100 ppm by weight). Electrical and physical properties of instruments and apparatuses are defined by the purity grade of the metal and articles made thereof.
A conventional method for producing high-purity niobium includes refining alumocalciothermic niobium having a starting niobium content of from 93 to 96 wt %, the refining being conducted in an electron beam furnace by a drip melt method into a crucible with electromagnetic stirring of the melt, the consumable preform being charged into a melting zone to draw an ingot. The ingot obtained after the remelting is used as a consumable preform for subsequent remelting. Required number of remelts is dictated by the content of impurities in the starting metal and the desired refinement degree. At least one of the remelts, except the last one, is conducted by successive overlaying of portions of the metal, upon the overlaying each of the portions being held with simultaneous exposure to the electron beam and electromagnetic stirring, and the next portion is overlaid after achieving a desired degree of refining the metal. The holding is executed after removing the consumable preform from the melting zone and terminating the drawing of the ingot. The final product is Hbi grade niobium meeting GOST 16099-80 standard which dictates the following content of each of the impurities: nitrogen, oxygen, carbon and aluminum at a level of 0.01 wt %, the total amount of tungsten and molybdenum impurities of 0.01 wt %, and tantalum amount of up to 0.1% wt (see RU patent No.2114928, publ. 10.07.98, Int.C1. C 22 B 34/24).
Niobium produced by the above method, however, comprises a total of 0.15 to 0.2 wt % impurities, i.e. twenty times the required amount.
Another conventional method for producing high-purity niobium includes electrolytic refining of a starting niobium from fluoride/chloride melts, followed by electron-beam melting of the cathode metal. The refining process includes anode dissolution of crude niobium in a melt comprising potassium fluoroniobate and an equimolar mixture of potassium and sodium chlorides to produce cathode metal having a relatively low content of refractory metal impurities (tungsten, molybdenum, tantalum), nitrogen and carbon. The following electron-beam melting provides essential reduction in the content of oxygen, iron, silicon and impurities of alkaline and alkaline-earth metals (Zelikman A. N. et al. Niobium and Tantalum, M., Metallurgy, 1991, pages 156-161).
However, the above method results in a rather high content of carbon (up to 0.02 wt %) and nitrogen (up to 0.05% wt %) impurities which are relatively slow removed in the electron-beam melting process (i.e. their removal requires additional remelts which is dictated not only by the increased evaporation losses of the metal and the longer melting cycle, but also by the increased concentration of difficultly volatile components), and a high content of tungsten and molybdenum impurities (up to 0.001 wt % each) which not only stay unremoved in the melting process, but also accumulate in the ingot owing to evaporation of the basic metal (niobium), and the greater the number of remelts, the greater the accumulation.
The object of the present invention is to provide a method for producing high-purity niobium having the total content of impurities in the range of from 0.002 to 0.007 wt % which would satisfy the requirements imposed on the materials used in microwave technology and microelectronics, with reduced niobium losses in both refining stages and increased yield of high-purity niobium.
In accordance with the invention, a method for producing high-purity niobium involves refining crude niobium in an electrolyte comprising a melt of salts including a complex niobium and potassium fluoride (potassium fluoroniobate) and an equimolar mixture of alkaline metal chlorides, said electrolyte further comprising sodium fluoride in the amount of from 5 to 15 wt %, and subjecting the obtained cathode deposit to electron-beam melting in a vacuum free of oil vapors at a residual gas pressure of from 5*10xe2x88x925 to 5*10xe2x88x927 mm Hg, a melting rate of from 0.7 to 2 mm/min and a leakage into a melting chamber of from 0.05 to 0.005 lxc2x7xcexcm/s to produce an ingot of niobium.
In a preferred embodiment, the electrolytic refining is carried out in a melt comprising the components in the following amount: 10-20 wt % potassium fluoroniobate, 5-15 wt % sodium fluoride, the balance being an equimolar mixture of potassium and sodium chlorides. In another preferred embodiment, the ingot produced after the electron-beam melting is subjected to plastic working at a temperature in the range of from 300 to 800xc2x0 C., and the obtained articles are subjected to thermal and chemical treatment.
The essence of the present invention is as follows. Sodium fluoride in the amount of from 5 to 15 wt % is added to an electrolyte comprising a complex niobium and potassium fluoride and an equimolar mixture of alkaline metal chlorides. This changes the discharge (dissolution) potential relationship of niobium and the majority of accompanying impurities (including N, C, W, Mo, Ta, Fe etc.) and provides a more fine purification of niobium.
Furthermore, the addition of sodium fluoride to the electrolyte promotes the formation of a protecting film of lower nickel fluorides on the internal surface of a working vessel, which reduces the internal surface wear and increases the life of the vessel.
The presence of sodium fluoride in the electrolyte in the range in accordance with the invention significantly reduces the electrolyte melting point, hence, the viscosity at the electrolytic refining temperature of 680-760xc2x0 C. The reduction in the electrolyte viscosity improves adhesion between the deposit and the cathode (i.e. prevents shedding the deposit on the bottom of the vessel) and substantially prevents entrainment of the electrolyte by the formed cathode deposit. Niobium obtained after termination of the electrolytic refining process is in the form of a coarse-dendrite cathode deposit, and the current efficiency for tetravalent niobium is raised to 90-98%. The anode metal output factor may be brought to 90% without sacrificing the quality of the produced metal.
The conditions of the electron-beam melting in accordance with the invention, in particular: melting the obtained cathode metal in a vacuum free of oil vapors at a residual gas pressure of from 5*10xe2x88x925 to 5*10xe2x88x927 mm Hg, a leakage into the melting chamber of from 0.05 to 0.005 lxc2x7xcexcm/s and a melting rate of from 0.7 to 2 mm/min, provide maximum removal of the impurities: oxygen (up to 0.0002 wt %), alkaline and alkaline-earth metals (up to 0.00001 wt %), iron and silicon (up to 0.00001 wt % each), and, at the same time, prevent the increase in the content of nitrogen and carbon impurities above their equilibrium values in niobium (0.0004 wt %) at a minimum number of remelts and minimum niobium losses associated with them.
Stringent requirements are imposed not only on the purity, but also on the metal structure of niobium articles used in microwave technology and microelectronics. These requirements are met owing to the claimed conditions of plastic working at a temperature from 300 to 800xc2x0 C. and subsequent thermal and chemical treatment of articles. The plastic working conditions in accordance with the invention provide the production of worked articles free of the microporosity inherent in the ingots, and exclude pollution of niobium by interstitial impurities (i.e. maintain the starting cast metal purity in the articles). This provides for the attainment of the desired service performance of the articles.
Addition of less than 5 wt % of sodium fluoride to the electrolyte comprising a complex niobium and potassium fluoride and an equimolar mixture of alkaline metal chlorides increases the viscosity of the electrolyte melt in the electrolytic refining process, raises the content of impurities of refractory metals, iron, nitrogen and carbon in the cathode metal, reduces the current efficiency for tetravalent niobium to 85%, and increases niobium losses in the subsequent electron-beam melting due to splashing caused by the increased content of electrolyte inclusions.
The increase in the sodium fluoride content in the electrolyte above 15% is unadvisable because this leads to the increased melting point of the electrolyte and disappearance of the effect of melt viscosity reduction at the electrolytic refining, and raises iron content in the cathode metal.
When the electron-beam melting of the obtained electrolytic niobium is conducted in a vacuum created by oil-vapor pumps, the carbon impurity content in the metal increases up to 0.005 wt % due to the increased content of hydrocarbons in the residual gas environment in the melting chamber.
The electron-beam melting under a residual gas pressure in the melting chamber above 5*10xe2x88x925 mm Hg leads to the enrichment of niobium by interstitial impurities, while the residual gas pressure in the melting chamber at a level of 5*10xe2x88x927 mm Hg provides the attainment of equilibrium concentrations of interstitial impurities in niobium; the melting at a lower residual gas pressure is unreasonable because this results in a longer melting cycle, increases the cost of electron-beam installations and makes their servicing more complicated.
With a leakage into the melting chamber above 0.05 1xc2x7xcexcm/s, niobium is enriched by interstitial impurities.
Reduction of the leakage value below 0.005 1xc2x7xcexcm/s is unadvisable as this results in the increased cost of the electron beam installations and makes their servicing more complicated.
Melting conducted at a rate below 0.7 mm/min results in the enrichment of niobium by impurities of refractory metals (tungsten, molybdenum, tantalum) due to evaporation of the basic metal, and additional evaporation losses of niobium.
Melting at a rate above 2 mm/min prevents the achievement of equilibrium concentrations of interstitial impurities and volatile impurities in niobium, i.e. leads to incomplete refining of niobium from these impurities.
Plastic working of the obtained high-purity niobium ingots at a temperature below 300xc2x0 C. fails to eliminate structural deficiencies (microporosity) in the ingots, which could result in high-voltage breakdown if the ingot is used to manufacture microwave cavities, or in splashing the metal and deterioration of the film quality if niobium is used as a magnetron sputtering target.
With plastic working of high-purity niobium at a temperature above 800xc2x0 C., the metal is polluted by interstitial impurities.